1. Field
This disclosure relates generally to coupling laser light in fibers and, more particularly, a method and apparatus for combining laser light in a fiber bundle.
2. Description of Related Art
In the fields of optical communication and lasers, particularly high power lasers, it is desirable to provide apparatus and methods for combining multiple optical sources into a single optical output and/or to provide multiple optical outputs from a single optical source. In this specification, the term “optical” is given the meaning used by those skilled in the art, that is, “optical” generally refers to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infrared and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibers.
Combining multiple optical sources into a single optical output having optical power nearly equal to the sum of the powers of the individual sources can be accomplished through the combination of the optical sources. One apparatus known in the art for combining N sources is a 1×N fiber coupler. U.S. Pat. No. 5,175,779, issued Dec. 29, 1992 to Mortimore, describes a 1×N single-mode star coupler configured to couple light into multiple fibers at two wavelengths. In Mortimore, multiple single mode fibers are stripped of their primary coating and constrained around a single central fiber, which is also a single mode fiber stripped of its primary coating. All fibers are inserted into a tight fitting silica base glass capillary tube. The fiber and the tube are heated and pulled to form a tapered coupler. During the pulling process, light transmitted through the central fiber and at least one of the multiple fibers disposed around the central fiber is measured. When the light in the central fiber and the fiber disposed around the central fiber is nearly equal at the two desired wavelengths, the pulling process is terminated.
The 1×N star coupler disclosed by Mortimore and other similar apparatus known in the art provide the capability to combine optical sources at relatively lower powers. Furthermore, as the optical power in each fiber is increased, this prior art has the disadvantage that the combined power must propagate in the core of the single central fiber. When the combined optical power is high, undesirable nonlinear effects in, or damage to, the single central fiber may occur. For example, at high optical powers, Stimulated Brillouin Scattering (SBS) may arise. This nonlinear optical effect-results from the interaction of the light in the central fiber with acoustic waves in the fiber medium through which the light is propagating, producing inelastic backscattering of the light with a frequency shift equal to the frequency of the acoustic waves. The backward propagating light is amplified at the expense of the forward propagating light. Further, the acoustic waves may also be amplified by this effect, generating an acoustic intensity that can easily damage the single central fiber.
Splitting a single optical source into multiple optical outputs may also be provided by the 1×N star coupler described above, but the power handling capabilities of the coupler are again limited by the single central fiber. Further, if the optical source is a single plane wave, additional optical devices are needed to couple the plane wave into the single central fiber.
Devices are known in the art which allow an optical plane wave to be coupled to multiple fibers without using a single central fiber. For example, U.S. Pat. No. 5,852,699, issued Dec. 22, 1998 to Lissotschenko et al., discloses a coupling element having an array of lenses where each lens focuses an incident light beam onto a specific fiber in a fiber bundle. Hence, the coupling element splits the incident plane wave into multiple light beams, each of which are directed to a separate optical fiber.
The coupling efficiency for coupling an optical plane wave into multiple fibers using the approach disclosed by Lissotschenko (or other similar techniques known in the art) is generally limited to about 30%. Even assuming perfect alignment, the coupling efficiency is limited by the packing of both the fibers in the fiber bundle and the lenses in the array of lenses. The coupling efficiency is further limited by clipping that occurs at the edge of each lens in the array. Finally, the coupling efficiency is reduced because the fiber modes only accept a Gaussian-profiled fraction of the input beam. Therefore, even though the optical plane wave may be a high power optical beam, a significant portion of that power is lost when coupling the beam into multiple fibers using apparatus and methods known in the art.
U.S. 5,408,556 to Wong discloses a 1×N splitter for single-mode optical fiber that includes an individual single-mode optical fiber having its junction end juxtaposed, through a focusing lens/junction element, to the end of a bundle of arbitrarily arranged single-mode fibers which are fused together along a portion of their lengths and which have a total diameter approximately equal to the diameter of the first single-mode fiber. The 1×N splitter is formed by trimming a limited portion of the cladding from and fusing together in a bundle a plurality of parallel but randomly arranged optical fibers at a fuse region with substantially uniform heat while controllably drawing all the fibers in the bundle at the same time monitoring crosstalk from a single input fiber to all output fibers to draw down the bundle size and to promote uniform crosstalk, then cleaving the fiber bundle at the fused region, abutting and aligning a single-mode optical fiber having cladding of substantially the same diameter as the fused bundle with the cleaved fused bundle and joining the single-mode optical fiber to the cleaved fused bundle with a spot weld which forms a focusing junction. The matched sizing and focusing junction minimizes return losses due to back reflection.
Therefore, there is a need in the art for a method and apparatus for combining the optical power of multiple optical fibers to provide a single high power optical output. There is also a need in the art for a method and apparatus for coupling an optical beam into multiple optical fibers at a greater coupling efficiency than other methods and apparatus known in the art. Further, there is a need in the art for efficiently coupling an optical beam in free space into multiple optical fibers, and coupling optical beams propagating in multiple optical fibers into free space.